Silicone pressure-sensitive adhesives

Abstract: A silicone pressure-sensitive adhesive composition having high adhesion and tack while having a high percentage of adhesive failure when peeled from substrates is disclosed, said composition comprising (A) 50 to 80 parts by weight of an MQ organopolysiloxane resin; (B) 20 to 50 parts by weight of a polydiorganosiloxane having at least 3 alkenyl groups in its molecule, with the proviso that the alkenyl group content of said polydiorganosiloxane is 0.1 to 4 mole percent, the total amount of components (A) and (B) being 100 parts by weight; (C) an organohydrogenpolysiloxane crosslinking agent having, on average, at least 3 SiH groups per molecule, the amount of said organohydrogenpolysiloxane being sufficient to provide a molar ratio of said SiH groups of component (C) to said alkenyl groups of component (B) in the range of 0.1 to <1.0; and (D) a curing amount of a hydrosilation catalyst.

Description: The present invention is a silicone pressure-sensitive adhesive which comprises a resin component, a polymer component having alkenyl groups in its molecule, a silicon hydride-functional crosslinking agent and a catalyst, characterized by a molar ratio of SiH groups of said crosslinking agent to alkenyl groups of said polymer being within the range of 0.1 to <1.0. Silicone pressure-sensitive adhesives (hereinafter PSAs) typically contain at least two primary components, namely a linear siloxane polymer and an MQ resin comprising triorganosiloxane (M) units (i.e., R 3 SiO 1/2 units, in which R denotes a monovalent organic group) and silicate (Q) units (i.e., SiO 4/2 units). In addition, silicone PSA compositions are generally provided with some crosslinking means in order to optimize various properties of the final adhesive product. Typically, in view of the high viscosity imparted by the polymer component, these PSA compositions are usually dispersed in an organic solvent for ease of application. When silicone PSA compositions employ an alkenyl-functional polymer and a crosslinking agent containing silicon-bonded hydrogen atoms, they are cured by a hydrosilation addition reaction using a platinum-type catalyst. In such systems, the molar ratio of silicon bonded hydrogen groups to silicon-bonded alkenyl groups is typically greater than 1, as exemplified by US-A 4,988,779. Although many of these PSAs have good adhesive properties, they often lack a balance between aggressive adhesion to substrates, such as metal and high initial tack. A major drawback of the art is that they tend to fail in a cohesive mode when peeled from a substrate, particularly when the peel rate is high. This has the untoward effect of transferring some of the adhesive to the substrate from which it is peeled (i.e., small "islands" of PSA remain on the substrate after the PSA is delaminated). This is undesirable in many commercial applications (e.g., PSA residues left on components in electronic masking applications). EP-A-0 506 370 discloses a solventless or high solids-containing organopolysiloxane composition curable to produce a pressure-sensitive adhesive having high tack and high peeladhesion, comprising (A) a toluene soluble, resinous copolymer comprising R 3 SiO 1/2 units; (B) an alkylen-terminated polydiorganosiloxane having a viscosity of from about 10 to about 500 centipoise at 25 °C; (C) from 0 to about 90 mole % of a hydride-terminated organohydrogenpolysiloxane having a viscosity of from about 10 to about 1000 centipoise at 25 °C; (D) from about 10 to about 100 mole % of a hydrogen-containing polydiorganosiloxane crosslinking agent; the combined average hydride equivalent weight of (C) and (D) being at least 1500 grams/silicon-bonded hydrogen atom; the mole percentages of (C) and (D) being based on the total silicon-bonded hydrogen content of (cC) and (D); the total of (B), (C) and (D) being from about 25 to about 50 parts by weight; the total of (A), (B), (C) and (D) being 100 parts by weight; the ratio of silicon-bonded hydrogen atoms in (C) and (D) to olefinically unsaturated radiclas in the total of (A) and (B) being in the range of from about 1.1:1 to about 15.0:1; (E) a catalytic amount of a hydrosilation catalyst; and (F) from 0 to about 40 % by weight of an organic solvent. US-A-5 169 727 discloses a pressure-sensitive adhesive composition comprising (a) a benzene soluble, resious copolymer, (b) a diorganoalkenylsiloxy endblocked polydiorganosiloxane, (c) a diorganohydrogensiloxy endblocked polydiorganosiloxane, (d) an organosiloxane crosslinking agent containing more than two groups that will react with a silicon-bonded hydrogen of a diorganoalkenylsiloxy endblocked polydiorganosiloxane by means of a hydrosilation reaction, said organosilicon compound being selected from organosiloxanes having 1 to 15 silicon atoms, and (e) a hydrosilation catalyst in an amount sufficient to effect curing of the composition. We have found that silicone PSA compositions having both high adhesion and high tack can be prepared from certain alkenyl-functional polymers in combination with crosslinkers containing silicon-bonded hydrogen characterized by the molar ratio of silicon-bonded hydrogen groups to silicon-bonded alkenyl groups being from 0.1 to <1.0. At the same time, the optimum PSAs according to our present invention exhibit at least a 90% adhesive failure mode when subjected to a standard peel test, described infra. Thus, the above mentioned adhesive transfer problem is either reduced or eliminated. Furthermore, it has surprisingly been found that the instant silicone PSAs have a lower adhesion to a fluorosilicone release liner than comparable systems, wherein the above mentioned molar ratio is at least 1, while still maintaining their aggressive adhesion to substrates such as steel. This latter characteristic facilitates release of our PSAs from typical fluorosilicone release liners such as those described in US-A 4,736,048. The present invention therefore provides a silicone pressure-sensitive adhesive comprising (A) 50 to 80 parts by weight of a organopolysiloxane resin soluble in either a hydrocarbon liquid or in a silicone liquid and consisting essentially of R 3 SiO 1/2 siloxane units and SiO 4/2 siloxane units wherein each R denotes a monovalent radical selected from hydrocarbon and halogenated hydrocarbon radicals, at least 1/3 of all R radicals being methyl and the mole ratio of R 3 SiO 1/2 siloxane units to SiO 4/2 siloxane units in the soluble organopolysiloxane having a value of from 0.6/1 to 1.2/1; (B) 20 to 50 parts by weight of a polydiorganosiloxane, the organic radicals thereof being selected from hydrocarbon and halogenated hydrocarbon radicals wherein at least 1/2 of said organic radicals are methyl, said polydiorganosiloxane having at least 3 alkenyl groups in its molecule, with the proviso that the alkenyl group content of said polydiorganosiloxane is 0.1 to 4 mole percent, the total amount of components (A) and (B) being 100 parts by weight; (C) an organohydrogenpolysiloxane crosslinking agent having, on average, at least 3 SiH groups per molecule, the amount of said organohydrogenpolysiloxane being sufficient to provide a molar ratio of said SiH groups of component (C) to said alkenyl groups of component (B) in the range of 0.1 to <1.0; and (D) a curing amount of a hydrosilation catalyst. The present invention further relates to a substrate having at least a portion of its surface coated with the above described pressure-sensitive adhesive composition. Component (A) of the present invention is a soluble, hydroxyl-functional organopolysiloxane resin consisting essentially of R 3 SiO 1/2 siloxane units and SiO 4/2 siloxane units. By the term soluble in either a hydrocarbon liquid or in a silicone liquid it is meant that the organopolysiloxane can be dissolved in either a hydrocarbon liquid, such as benzene, toluene, xylene and heptane or in a silicone liquid such as cyclic or linear polydiorganosiloxanes. Preferably the resin is soluble in component (B), delineated below. In the formula for component (A), R denotes a monovalent radical selected from hydrocarbon and halogenated hydrocarbon radicals, preferably having less than 20 carbon atoms, and most preferably having from 1 to 10 carbon atoms. Examples of suitable R radicals include alkyl radicals, such as methyl, ethyl, propyl, pentyl, octyl, undecyl and octadecyl; cycloaliphatic radicals, such as cyclohexyl; aryl radicals such as phenyl, tolyl, xylyl, benzyl, alpha-methyl styryl and 2-phenylethyl; alkenyl radicals such as vinyl; and chlorinated hydrocarbon radicals such as 3-chloropropyl and dichlorophenyl. To enhance the solubility of component (A) in component (B), it is desirable to select the predominant organic radicals of the former to match the predominant organic radicals of the latter. Preferably, at least one-third, and more preferably substantially all R radicals in the formula for component (A), are methyl radicals. The methyl radicals can be distributed in any desired arrangement among the R 3 SiO 1/2 siloxane units; however, it is preferred that each R 3 SiO 1/2 siloxane unit bear at least one, and more preferably at least two, methyl radicals. Examples of preferred R 3 SiO 1/2 siloxane units include Me 3 SiO 1/2 , PhMe 2 SiO 1/2 and Ph 2 MeSiO 1/2 where Me denotes methyl radical and Ph denotes phenyl radical. Component (A) includes a resinous portion wherein the R 3 SiO 1/2 siloxane units (i.e., M units) are bonded to the SiO 4/2 siloxane units (i.e., Q units), each of which is bonded to at least one other SiO 4/2 siloxane unit. Some SiO 4/2 siloxane units are bonded to hydroxyl radicals resulting in HOSiO 3/2 units (i.e., TOH units), thereby accounting for the silicon-bonded hydroxyl content of the organopolysiloxane, and some are bonded only to other SiO 4/2 siloxane unit. In addition to the resinous portion, component (A) can contain a small amount of a low molecular weight material comprised substantially of a neopentamer organopolysiloxane having the formula (R 3 SiO) 4 Si, the latter material being a byproduct in the preparation of the resin according to the methods of US-A 2,676,182. For the purposes of this invention, component (A) consists essentially of R 3 SiO 1/2 siloxane units and SiO 4/2 siloxane units in a molar ratio of 0.6 to 1.2, respectively. It is preferred that the mole ratio of the total M siloxane units to total Q siloxane units of (A) be between 0.8 and 1.1, most preferably 0.9 to 1.0. The above M/Q mole ratios can be easily obtained by 29 Si nuclear magnetic resonance (NMR), this technique being capable of a quantitative determination of the molar contents of: M (resin), M(neopentamer), Q (resin), Q(neopentamer) and TOH. For the purposes of our invention, the M/Q ratio {M(resin) + M(neopentamer)}/{Q(resin) + Q(neopentamer)} represents the ratio of the total number of triorganosiloxy groups of the resinous and neopentamer portions of component (A) to the total number of silicate groups of the resinous and neopentamer portions of (A). The above definition of the M/Q ratio applies to resins which contain neopentamer as a byproduct of preparation and no further addition of neopentomer is contemplated. Further, when the resin (A) is prepared by another method (e.g., by co-hydrolysis and condensation of alkoxysilanes), neopentamer may not be present and the M/Q ratio would then be defined in terms of only the resinous portion. It is further preferred that the resinous portion of component (A) have a number average molecular weight (M n ) of 2,000 to 6,000, particularly 2,300 to 3,300, when measured by gel permeation chromatography (GPC), the neopentamer peak, if present, being excluded from the measurement. In this molecular weight determination, narrow fractions of MQ resins are used to calibrate the GPC equipment, the absolute molecular weights of the fractions being first ascertained by a technique such as vapor phase osmometry. Component (A) can be prepared by any method providing said method provides a soluble organopolysiloxane consisting essentially of R 3 SiO 1/2 siloxane units and SiO 4/2 siloxane units. It is preferably prepared by the silica hydrosol capping process of US-A 2,676,182; as modified by US-A 3,627,851 and US-A 3,772,247; which teach how to prepare soluble organopolysiloxanes which meet the requirements of the present invention. Briefly stated, this modified process when used to prepare the preferred component (A) of our invention, comprises limiting the concentration of the sodium silicate solution, and/or the silicon-to-sodium ratio in the sodium silicate, and/or the time before capping the neutralized sodium silicate solution, to generally lower values than those disclosed by US-A 2,676,182 alone. This prevents excessive growth of the silica particles and produces a soluble organopolysiloxane having the preferred M n . Thus, the preferred silicate concentration is generally limited to a value of 40 to 120, preferably 60 to 100, and most preferably about 75 grams/liter. The neutralized silica hydrosol is preferably stabilized with an alcohol, such as isopropanol, and capped with R 3 SiO 1/2 siloxane units as soon as possible, preferably within 30 seconds, after being neutralized. The sodium silicate employed preferably has the formula Na 2 O· x SiO 2 , wherein x has a value of 2 to 3.5. The level of the silicon bonded hydroxyl groups on the organopolysiloxane component (A), may be reduced, preferably to less than 1 weight percent. This may be accomplished by reacting hexamethyldisilazane with the organopolysiloxane. Such a reaction may be catalyzed with trifluoroacetic acid. Alternatively, trimethylchlorosilane or trimethylsilylacetamide may be reacted with the organopolysiloxane, a catalyst not being necessary in this latter case. Component (B) of our invention is a polydiorganosiloxane in which the organic radicals are selected from hydrocarbon and halogenated hydrocarbon radicals. For the purposes of the present invention, at least 1/2 of these organic radicals are methyl and the polydiorganosiloxane has, on average, at least 3 alkenyl groups in its molecule, with the proviso that its alkenyl group content is 0.1 to 4 mole percent, preferably 0.1 to 0.5 mole percent. This latter quantity may be calculated according to the following equation: % alkenyl content = (Alk)/(Silox) x 100 wherein Alk = moles of alkenyl radicals in the polydiorganosiloxane and Silox = total moles of siloxane units in the polydiorganosiloxane. When the alkenyl content is below 0.1 mole percent, optimum adhesion and tack properties can not be obtained at a molar ratio of silicon-bonded hydrogen groups to silicon-bonded alkenyl groups of less than 1.0. On the other hand, when the alkenyl content is more than 4 mole percent, more cohesive failure is observed and it is difficult to maintain a good balance between adhesion and tack while and still attaining a level of adhesive failure of at least 90%. Preferably, component (B) is a polymer or copolymer having the general formula R 1 R 2 2 SiO(R 2 2 SiO) n SiR 2 2 R 1 wherein each R 2 is a monovalent radical independently selected from hydrocarbon and halogenated hydrocarbon radicals and each R 1 denotes a radical selected from R 2 radicals and an OH radical. The R 2 radicals may be selected from the saturated hydrocarbon and halogenated hydrocarbon radicals delineated above for R or alkenyl radicals having 2 to 10 carbon atoms. Examples include vinyl, allyl, butenyl, hexenyl, cyclohexenyl and beta-cyclohexenylethyl. Preferably, the alkenyl radicals are vinyl, hereinafter also represented as Vi. On average, at least three R 2 radicals in the above formula are alkenyl radicals. Of course, the average value of n in the above formula is such that it comports with the requirements of alkenyl group content. Thus if polydiorganosiloxane (B) has only 3 alkenyl radicals in its molecule, the minimum permissible degree of polymerization (DP), including terminal siloxane units, may be calculated from equation (i): 4% = (3)/DP x 100 such that DP = 75. It is preferred that the average DP of polydiorganosiloxane (B) is 500 or greater. Component (B) can be comprised of a single polydiorganosiloxane or a mixture of two or more different polydiorganosiloxanes. It is preferred that at least 85% of the R 2 radicals of component (B) are methyl radicals, which radicals can be distributed in any manner in the polydiorganosiloxane. Further, component (B) can comprise trace amounts of siloxane branching sites provided that it remains flowable. Component (C) of the present invention is at least one organohydrogenpolysiloxane crosslinking agent having, on average, at least 3 SiH groups per molecule. Thus, component (C) can be a mixture of an organohydrogenpolysiloxane having only 2 SiH groups per molecule with one or more organohydrogenpolysiloxanes having more than 3 SiH groups per molecule, as long as the final crosslinking agent has, on average, at least 3 SiH groups per molecule. When component (C) contains an average of fewer than 3 SiH groups per molecule, the adhesive properties of the PSAs prepared therewith are inferior, particularly when the number average molecular weight of the resin (A) is greater than 3,500. Component (C) may be an SiH-functional resin, similar to those described supra as component (A) wherein at least some of the M units have been replaced with siloxane units of the structure R 2 HSiO 1/2 , in which R has its previously defined meaning. Preferably, component (C) is a linear siloxane having the formula or wherein R 3 is independently selected from alkyl radicals having 1 to 4 carbon atoms, such as methyl, ethyl and propyl; a phenyl radical; a trifluoropropyl radical; and a chloropropyl group. In formulas (I) and (II), y has an average value of 3 to 40 and z has an average value of 1 to 100. The above described organohydrogenpolysiloxanes are well known in the art and many of these compounds are available commercially. In addition to the above described materials, component (C) may be selected from the crosslinking agents disclosed in U.S. Patent No. 5,290,885. Briefly stated, these crosslinking agents comprise a reaction product of an organohydrogenpolysiloxane (I) and an unsaturated organic compound (II). The organohydrogenpolysiloxane (I) is either a cyclic siloxane having the formula or a linear siloxane having the formula or wherein R 3 is independently selected from alkyl radicals having 1 to 4 carbon atoms, such as methyl, ethyl and propyl; a phenyl radical; a trifluoropropyl radical; and a chloropropyl group. In formulas (i), (ii) and (iii), j has an average value of 4 to 8, k has an average value of 5 to 20 and n has an average value of 1 to 20, with the proviso that k + n ≤ 25. In preferred cyclic organohydrogenpolysiloxanes, each R 3 is matched with the organic radicals of components (A) and (B) and the average value of j is 5 to 7, most preferably 5. The preferred linear organohydrogenpolysiloxanes are represented by formula (ii) wherein k is 5 to 10 and each R 3 is similarly matched. The unsaturated organic compound (II) can be a linear or branched alpha-alkene having 6 to 28 carbon atoms. This compound is preferably selected from linear alkenes having the formula CH 2 =CH 2 (CH 2 ) m CH 3 wherein m is 5 to 25, more preferably 9 to 25 and most preferably 9 to 15. Specific examples of preferred alpha-alkenes include 1-decene, 1-dodecene and 1-octadecene. Alternatively, (II) can be an aromatic compound having the formula Ph-R 4 , wherein R 4 is a terminally unsaturated monovalent hydrocarbon group having 2 to 6 carbon atoms. Preferably, unsaturated organic compound (II) is alpha-methylstyrene. In order to prepare the above described crosslinking agents, organohydrogenpolysiloxane (I) and unsaturated organic compound (II) are reacted using a platinum metal-type catalyst to promote the hydrosilation addition of the SiH functionality of the former to the unsaturation of the latter. This reaction is well known in the art and is typically conducted at temperatures in the range of 100 to 150°C., either neat or in the presence of an inert organic solvent such as toluene or hexane. For the purposes of our invention, the relative amounts of components used to form the crosslinking agent are such that, on a theoretical basis, at least 10 mole percent of the original SiH functionality present in the organohydrogenpolysiloxane (I) is reacted with the unsaturated organic compound (II) and the final reaction product contains 3 to 15 residual SiH groups. It is preferred that the final crosslinking agent of this type contain 4 to 6 residual SiH groups per molecule and the ratio of (II) to (I) to be used in the above reaction can be calculated to approximate such a result. Component (C) is added in an amount sufficient to provide from 0.1 to <1.0, preferably from 0.2 to 0.6, silicon-bonded hydrogen atoms for each alkenyl radical in polydiorganosiloxane (B). The optimum amount of this crosslinking agent may be determined by routine experimentation using the data graphed in Figures 1 and 2 as a guideline. Figure 1 is a plot of the molar vinyl group content of our PSA polymer versus the molar ratio of silicon-bonded hydrogen groups to silicon-bonded vinyl groups for optimum adhesive strength. Figure 2 is a plot of the molar vinyl group content of our PSA polymer versus the molar ratio of silicon-bonded hydrogen groups to silicon-bonded vinyl groups for optimum tack. Our PSA compositions additionally contain a platinum group metal-containing hydrosilation catalyst (D). These catalysts are the well known platinum and rhodium catalysts which are effective for catalyzing the reaction of silicon-bonded hydrogen atoms with silicon-bonded alkenyl radicals. In addition, complexes of the metals ruthenium, palladium, osmium and irridium can be utilized. A preferred platinum-containing catalyst is a chloroplatinic acid-vinylsiloxane complex disclosed by US-A 3,419,593. Such platinum group metal-containing catalysts accelerate the reaction of component (B) with the crosslinking agent (C) and permit room temperature or low temperature curing of the composition. The platinum group metal-containing catalyst is added in an amount sufficient to provide 0.1 to 1,000, preferably 1 to 500 and most preferably 10 to 300, parts by weight of platinum for each one million weight parts of the composition. It is recommended that the instant PSA compositions also include a platinum catalyst inhibitor. Preferred inhibitors include various "ene-yne" systems, such as 3-methyl-3-pentene-1-yne and 3,5-dimethyl-3-hexene-1-yne; acetylenic alcohols, such as 3-methyl-1-butyne-3-ol, 3,5-dimethyl-1-hexyne-3-ol, 3-methyl-1-pentyne-3-ol, and phenylbutynol; maleates and fumarates, such as dialkyl, dialkenyl and dialkoxyalkyl fumarates and maleates; cyclovinylsiloxanes; and benzyl alcohol. The platinum catalyst inhibitor can be used in any amount that will retard the catalyzed addition reaction at room temperature while not preventing said reaction at elevated temperature. The exact proportion of this ingredient to be used may be readily determined by routine experimentation. In general, small amounts of additional ingredients may be added to the compositions of this invention. For example, antioxidants, pigments, stabilizers and fillers may be added as long as they do not materially reduce the pressure sensitive adhesive properties of these compositions. Compositions of this invention can be prepared by homogeneously mixing 50 to 80 parts by weight of component (A) and 20 to 50 parts by weight of component (B) in the presence of a non-reactive solvent (i.e., a total of 100 weight parts of resin and polymer). Preferably, from 65 to 75 parts of (A) per 100 parts by weight of (A) + (B) are used. Useful solvents include hydrocarbons, such as toluene, xylene, heptane, and mineral spirits; volatile siloxanes, such as octamethylcyclotetrasiloxane and hexamethyldisiloxane; halohydrocarbons, alcohols, esters, ketones and combinations of these solvents. The amount of solvent required depends on the viscosity of the polydiorganosiloxane (B). Higher viscosity polydiorganosiloxane polymers require more solvent than lower viscosity polymers to facilitate the preparation, handling and application of the compositions. However, when polydiorganosiloxane (B) has a low DP (e.g., less than about 200), a solventless PSA can be obtained. Suitable mixing means for preparing the instant compositions include a spatula, a drum roller, a mechanical stirrer, a three-roll mill, a sigma blade mixer, a bread dough mixer, and a two-roll mill. Component (C) may be mixed with (A) and (B) and the platinum catalyst inhibitor can also be incorporated in this mixture. The platinum group metal-containing catalyst is preferably added last, or just prior to use. After application to a substrate, curing of the compositions of this invention can be accomplished by heating at temperatures of up to 300°C., preferably at 80 to 150°C., for a suitable length of time. Alternatively, when an inhibitor is also used, the PSAs of this invention can be cured by exposure to ultraviolet radiation. The compositions of our invention find utility as pressure sensitive adhesives and will readily stick to a solid support, whether flexible or rigid. These compositions may be applied to a surface by any suitable means such as rolling, spreading or spraying and then can be cured thereon. The surface of the support and the substrate to which the support is adhered may be any known solid material such as metals, like aluminum, silver, copper, iron and their alloys; porous materials such as paper, wood, leather, and fabrics; organic polymeric materials such as polyolefins, such as polyethylene and polypropylene; fluorocarbon polymers such as polytetrafluoroethylene and polyvinylfluoride; polystyrene; polyamides such as nylon; polyesters and acrylic polymers; painted surfaces; siliceous materials such as concrete, bricks, cinderblocks and glass such as glass cloth. Porous materials such as glass cloth are often impregnated with a substance that will prevent the migration of the silicone pressure sensitive adhesive from one surface to another surface of the support. In this regard, it is also well known to chemically treat the surface of a flourocarbon polymer support to enhance the adhesion of the silicone pressure sensitive adhesive to said surface. Useful articles which can be prepared with the silicone pressure sensitive adhesives of this invention include pressure sensitive tapes, labels, transfer films, emblems and other decorative or informational signs. An especially useful article is one comprising a flexible or rigid support that can withstand extreme temperatures, hot and/or cold, and bearing on at least one surface thereof the silicone pressure sensitive adhesives of this invention. Such an article makes full use of the stability at high temperatures and the flexibility at low temperatures that the silicone pressure sensitive adhesives of this invention possess. The following examples are presented to further illustrate the compositions of this invention, but are not to be construed as limiting the invention, which is delineated in the appended claims. All parts and percentages in the examples are on a weight basis and all measurements were obtained at 25°C., unless indicated to the contrary. The following components were used in the examples; Me and Vi represent methyl radical and vinyl radical, respectively. Polymers The following dimethylvinylsiloxy-terminated polydimethylsiloxane polymers having an average degree of polymerization (DP) of approximately 9,500 were used in the examples of the instant invention. These polymers had additional vinyl functionality (as methylvinylsiloxy units) randomly distributed along their main chains so as to provide the total molar vinyl contents indicated in the following table: Polymer Total Molar Vinyl Content of Polymer (%) Polymer 1 0.1 Polymer 2 0.15 Polymer 3 0.5 Polymer 4 2.0 Polymer 5 4.0 In addition, the following polydimethylsiloxane polymers were used to prepare comparative PSAs: Polymer 6 is a polydimethylsiloxane similar to Polymers 1 through 5 wherein the molar vinyl group content is 0.03%. Polymer 7 is a dimethylvinylsiloxy-terminated polydimethylsiloxane (i.e., no pendant vinyl functionality) having a molar vinyl content of 0.02%. Polymer 8 is a dimethylvinylsiloxy-terminated polydimethylsiloxane having a total average degree of polymerization (DP) of about 23 and having a total molar vinyl content of 9.6%. This polymer is essentially identical to the vinyl-functional polymer described in Example 15 of EP-A 0506372. Resins Resin 1 = a 72% solution in xylene of a solid MQ resin comprising trimethylsiloxy units and SiO 4/2 units in a molar ratio of 0.63:1 and having a silicon-bonded hydroxyl content of 3.5 weight percent (solids basis) and a number average molecular weight (M n ) of 5,000. Resin 2 = an 81% solution in xylene of a solid MQ resin comprising trimethylsiloxy units and SiO 4/2 units in a molar ratio of 1.1:1 and having a silicon-bonded hydroxyl content of 3.3 weight percent (solids basis) and M n of 2,700. Resin 3 = a 77% solution in xylene of Resin 2, wherein the resin has been capped with trimethylsiloxy groups so as to provide a residual silicon-bonded hydroxyl content of less than 1/2 weight percent (solids basis). The number average molecular weight of the above described resins was determined by gel-permeation chromatography (GPC) using Varian TSK 4000 + 2500 columns at 35°C, a chloroform mobile phase at 1 mL/min and an IR detector set at 9.1 micrometers to detect Si-O-Si. The GPC was calibrated using narrow fractions of similar resins as standards. The Mn values reported herein exclude any neopentamer, (Me 3 SiO) 4 Si, present in the resin component. The trimethylsiloxy/SiO 4/2 ratio of the resins was determined by 29 Si NMR and, in this case, the reported results include any neopentamer component present in the resin. Crosslinkers Crosslinker 1 = an organohydrogenpolysiloxane having the general formula Me 3 SiO(Me 2 SiO) 12 (MeHSiO) 28 SiMe 3 . Crosslinker 2 = an organohydrogenpolysiloxane having the general formula HMe 2 SiO(Me 2 SiO) 13 SiMe 2 H. Crosslinker 3 = an organohydrogenpolysiloxane having the general formula HMe 2 SiO(Me 2 SiO) 13 (MeHSiO)SiMe 2 H. Catalyst Catalyst 1 = a chloroplatinic acid complex of divinyltetramethyldisiloxane diluted with dimethylvinylsiloxy endblocked polydimethylsiloxane to provide 0.65 weight percent platinum prepared according to Example 1 of US-A 3,419,593. Example 1 Silicone PSA solutions were prepared by thoroughly blending Resin 3 with Polymer 4 in various weight ratios. Each blend was combined with sufficient Crosslinker 1 to provide a molar ratio of SiH groups (of the crosslinker) to SiVi groups (of the polymer) of 0.5. Each PSA solution was mixed with 0.6% of a diethyl fumarate (DEF) inhibitor and 0.6% of Catalyst 1, the proportions of the latter two components being based on the total weight of Resin 3 (solution) and Polymer 4. Each of the adhesive formulations was cast on 0.025 mm (1.0 mil) thick KAPTON™ polyimide sheets, the adhesive was cured by heating the coated sheets at 130°C. for 4 minutes to provide cured PSA film having a thickness of 0.038-0.05 mm (1.5 - 2 mils). The coated sheets were cut to prepare PSA tapes which were then evaluated according to the following procedures. The results are presented in Table 1. Adhesion (A) Adhesion was measured by cutting the adhesive-coated sheets into one inch (25.4 mm) wide tape, rolling this pressure-sensitive tape onto a stainless steel panel with a five pound (2,270 g) roller and then pulling the tape from the panel at a 180 degree angle at a rate of 12 inches per minute (305 mm/min). The average force required to peel each film from the panel is reported in grams/millimeter (gm/mm). Percent Adhesive Failure (AF) When the above described adhesion test resulted in some adhesive transfer from the tape to the steel panel (i.e., some cohesive failure), the AF value represents the percent of the adhesive film remaining on the tape (i.e., AF = 100% when no adhesive transferred to the panel; AF = 0% when all of the adhesive transfers to the steel panel; and intermediate values between these extremes were determined visually). Tack Probe tack was measured on one-inch (25.4 mm) squares of the Kapton™-backed adhesive using a POLYKEN (R) probe tack tester, available from Testing Machines, Inc., Amityville, NY. The tack tester has a 0.5 cm diameter stainless steel probe. The test procedure used a 20 gram weight, a dwell time of 0.5 seconds and a pull speed of 0.5 cm per second. The results are reported as average tack, expressed in grams. R/P ratio Adhesion (gm/mm) AF (%) Tack (grams) 66/34 321.7 100 784 67/33 482.6 100 966 68/32 464.7 100 870 69/31 500.5 100 676 71/29 572.0 100 548 73/27 607.7 0 81 75/25 250.2 - 536.2 0 0 77/23 357.5 - 572.0 0 0 Based on the above results, it was determined that the optimum resin to polymer weight ratio (R/P) for these PSAs was 67/33. This represents a balance of optimum adhesion as well as tack values wherein the adhesive failure was at least 90%. Another series of PSAs was prepared as described above wherein the R/P weight ratio was 67/33 and the SiH/SiVi molar ratio was varied. The cured systems were evaluated as described above and the results are presented in Table 2. SiH/SiVi Adhesion (gm/mm) AF (%) Tack (grams) 0.1 929.4 70 1143 0.18 643.5 95 1075 0.25 616.7 100 1168 0.5 429.0 100 869 0.75 321.7 100 583 1.0 286.0 100 765 2.0 187.7 100 121 10.0 35.8 100 185 From Table 2 it can be seen that the optimum PSA composition with respect to adhesion, wherein the adhesive failure is at least 90%, occurs at an SiH/SiVi ratio of 0.18. Likewise, the optimum PSA composition with respect to tack occurs at an SiH/SiVi ratio of 0.25. The above described procedures were used to formulate PSAs wherein other polymers were substituted for Polymer 4, also at a resin/polymer ratio of 67/33. The corresponding optimum SiH/SiVi molar ratios with respect to adhesion (at least 90% adhesive failure) and tack were again obtained and are shown in Table 3. Polymer Optimum Molar SiH/SiVi Ratio Based on Adhesion Based on Tack Polymer 1 0.9 0.75 Polymer 2 0.5 0.75 Polymer 3 0.25 0.25 Polymer 5 0.18 0.18 Polymer 6 10 10 Polymer 7 30 30 The results of Table 3 are also plotted in Figures 1 and 2, for adhesion and tack, respectively. In these figures the log of the optimum SiH/SiVi ratio is plotted on the abscissa and the log of the molar vinyl group content of the polymers is plotted on the ordinate, the values for Polymer 4 being included in the graphs. It can be seen that the two plots exhibit similar trends, indicating that approximately the same relationship holds for the adhesion optimum and the tack optimum. It can further be seen that polymers having less than 0.1 mole percent vinyl content (e.g., Polymers 6 and 7) required an SiH/SiVi ratio considerably more than 1.0 to achieve optimum adhesion (at least 90% adhesive failure) and optimum tack values. Examples 2 - 9 Silicone PSAs were prepared as described above using Resin 1 as the resin component and Polymer 2 and Polymer 8 as the polymer components in the proportions shown in Table 4. In each case, the crosslinker was added so as to provide an SiH/SiVi molar ratio of 0.9. These PSAs were cured and tested as described above and the results are also presented in Table 4, wherein R/P represents the resin to polymer weight ratio (based on solids), A represents the adhesive strength (gm/mm), AF represents the percent adhesive failure and T represents the tack value (grams). Example Polymer R/P Crosslinker A AF T 2 Polymer 2 57/43 Crosslinker 3 429.0 100 394 3 Polymer 2 57/43 Crosslinker 2 178.7 60 246 4 Polymer 8 57/43 Crosslinker 3 178.7 100 302 5 Polymer 8 57/43 Crosslinker 2 125.1 20 282 6 Polymer 2 60/40 Crosslinker 3 697.1 100 541 7 Polymer 2 60/40 Crosslinker 2 518.3 0 607 8 Polymer 8 60/40 Crosslinker 3 286.0 100 227 9 Polymer 8 60/40 Crosslinker 2 178.7 20 212 From Table 4 it is seen that at an SiH/SiVi molar ratio of 0.9, the PSAs which employ Polymer 8 consistently exhibited inferior adhesive properties relative to those based on Polymer 2. This illustrates the advantage of the instant PSA compositions relative to those suggested in EP-A 0506372, which employ an essentially identical vinyl-functional polymer at the above mentioned SiH/SiVi ratio (Example 15 of EP-A 0506372). It can also be seen from Table 4 that the use of a crosslinker having at least 3 SiH groups in its molecule (i.e., Crosslinker 3) results in improved adhesive character relative to the crosslinker having only 2 SiH groups. The procedures of Examples 2 - 9 were followed wherein Resin 3 was used instead of Resin 1. The resulting PSAs were cured and evaluated as before and the results are shown in Table 5. Example Polymer R/P Crosslinker A AF T 10 Polymer 2 67/33 Crosslinker 3 893.7 98 731 11 Polymer 2 67/33 Crosslinker 2 1143.9 70 867 12 Polymer 8 67/33 Crosslinker 3 250.2 100 272 13 Polymer 8 67/33 Crosslinker 2 35.8 30 242 As in above Examples 2 - 9, the advantage of using Polymer 2 over Polymer 8 when the SiH/SiVi molar ratio is below 1.0 (i.e., 0.9) is apparent from the data presented in Table 5. The results again illustrate the benefit of using a crosslinker having at least 3 SiH groups in its molecule since the % adhesive failure dropped below 90% when the crosslinker had only 2 such groups.

Claims: A silicone pressure sensitive adhesive composition comprising: (A) 50 to 80 parts by weight of a organopolysiloxane resin soluble in either a hydrocarbon liquid or in a silicone liquid and comprising R 3 SiO 1/2 siloxane units and SiO 4/2 siloxane units wherein each R denotes a monovalent radical selected from hydrocarbon and halogenated hydrocarbon radicals, at least 1/3 of all R radicals being methyl and the mole ratio of R 3 SiO 1/2 siloxane units to SiO 4/2 siloxane units in the soluble organopolysiloxane having a value of from 0.6/1 to 1.2/1; (B) 20 to 50 parts by weight of a polydiorganosiloxane having the general formula R 1 R 2 2 SiO(R 2 2 SiO) n SiR 2 2 R 1 wherein each R 2 is a monovalent organic radical independently selected from hydrocarbon and halogenated hydrocarbon radicals and each R 1 denotes a radical selected from R 2 radicals and an OH group, wherein at least 1/2 of the organic radicals are methyl and said polydiorganosiloxane has at least 3 alkenyl groups in its molecule, with the proviso that the alkenyl group content of said polydiorganosiloxane is 0.1 to 4 mole percent and the total amount of components (A) and (3) is 100 parts by weight; (C) an organohydrogenpolysiloxane crosslinking agent having at least 3 SiH groups in its molecule selected from polymers having the formula and copolymers having the formula wherein R 3 is independently selected from alkyl radicals having 1 to 4 carbon atoms, a trifluoropropyl radical and a chloropropyl group, y has an average value of 3 to 40 and z has an average value of 1 to 100, the amount of said organohydrogenpolysiloxane being sufficient to provide a molar ratio of said SiH groups of component (C) to said alkenyl groups of component (B) of 0.1 to <1.0; and (D) a curing amount of a hydrosilation catalyst. The composition according to claim 1, wherein R of said resin (A) is methyl and said polydiorganosiloxane (B) is an alkenyl-functional polydimethylsiloxane. The composition according to claim 2 wherein the alkenyl groups on said polydiorganosiloxane (B) are vinyl radical. The composition according to claim 3 wherein the ratio of said resin (A) to said polydiorganosiloxane (B) is 65/35 to 75/25 and the number average molecular weight of said resin (A) is 2,300 to 3,300. A substrate having at least a portion of its surface coated with the pressure-sensitive adhesive composition of claim 1. The composition according to claim 1, wherein said crosslinking agent (C) is a hydrosilation reaction product of (I) an organohydrogenpolysiloxane selected from a cyclic siloxane having the formula a linear siloxane having the formula and a linear siloxane having the formula wherein R 3 is independently selected from alkyl radicals having 1 to 4 carbon atoms, a phenyl radical, a trifluoropropyl radical and a chloropropyl group, j has an average value of 4 to 8, k has an average value of 5 to 20 and n has an average value of 1 to 20, with the proviso that k + n < 25; and (II) an unsaturated organic compound selected from an alpha-alkene having 6 to 28 carbon atoms and an aromatic compound having the formula Ph-R 4 , wherein Ph represents a phenyl radical and R 4 is a terminally unsaturated monovalent hydrocarbon group having 2 to 6 carbon atoms.

Inventors: SCHMIDT RANDALL GENE, VINCENT GARY ALLEN, SCHMIDT, RANDALL GENE, VINCENT, GARY ALLEN
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1994
Main classification: C09J183
IPC codes: C09J183